Problem: Factor completely. $16-49x^2=$
Answer: Both $16$ and $49x^2$ are perfect squares, since $16=({4})^2$ and $49x^2=({7x})^2$. $16-49x^2 = ({4})^2-({7x})^2$ So we can use the difference of squares pattern to factor. ${a}^2 - {b}^2 =({a}+{b})({a}-{b})$ In this case, ${a}={4}$ and ${b}={7x}$ : $({4})^2 - ({7x})^2 =({4}+{7x})({4}-{7x})$ In conclusion, $16-49x^2=(4+7x)(4-7x)$ Remember that you can always check your factorization by expanding it.